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GUNS Magazine December 2012 Digital Edition - Page 52
if if you you shoot, shoot, it’ll it’ll hAppen, hAppen, but but you you cAn cAn effects And your minimize its prolonG its effects And your tube’s tube’s life. life. John barsness bout 15 years ago, Melvin Forbes of New Ultra Light Arms went prairie dog shooting for the first time with a few of his western friends. On a warm July afternoon the barrel of his .223 almost started to glow from dozens of 50-grain bullets zipping out the muzzle. One of the other guys said, “Hey, Melvin, you’re gonna fry that barrel!” A Melvin looked at him and asked, “What day of the week is it?” The guy looked puzzled. “Saturday.” Melvin smiled. “I do believe Douglas will be making barrels again on Monday!” While that glass-is-half-full statement is obviously true, many of us like to extend the life of exceptionally accurate barrels. To do that we need to understand how barrels burn out. Luckily, the subject has been studied considerably; mostly because of the money it costs to re-barrel military small arms and artillery. The erosion of a rifle bore can be broken down into three distinct but interrelated processes: thermal, chemical and mechanical. Thermal erosion is primarily caused by the heat of burning powder gases, the reason the bore right in front of the chamber suffers the most: As the gases produced expand while the bullet travels down the bore, their temperature drops. Chemical erosion occurs when some of the gases produced by burning smokeless powder essentially alter the alloy of the bore’s surface. Hydrogen and oxygen lower its melting point, but nitrogen actually tends to strengthen steel. Mechanical erosion is due to each bullet wiping away a slight amount of steel, or breaking off tiny pieces of an eroded bore. Heat checking, the “alligator The .220 Swift (above) is a great varmint round, especially for woodchucks and coyotes, but has lost favor for high-volume prairie dog shooting because it’s really hard on barrels, especially when shot hot. Sitting in the shade not only keeps this prairie dog shooter (below) cool but prevents the barrel of his Remington 700 VTR from heating up quite so quickly. One recent trend in varmint rifle stocks is holes in the fore-end to help air circulate around the barrel. skin” appearance of a fried throat, is primarily caused by steel turning brittle as the surface cools after each shot. The difference in flexibility between the hardened surface and the softer steel underneath results in tiny cracks. At the same time, tiny amounts of softer alloying elements are melted away, like water evaporating from a mud puddle. The result is gator skin. Heat-checked steel is even more susceptible to chemical and thermal erosion, since the cracks cause more turbulence in the expanding gases. Instead of streaming down the bore, hot gas circulates longer in the eroded area. Cracks at right angles to the bore are most affected, since they “catch” far more hot gas. Roughly machined bores, with obvious reamer marks, also probably erode faster than very smooth barrels, due to the same effect. A cracked bore is also more vulnerable to the primary mechanical cause of erosion, the “melt-wipe” process. The roughness of a heatchecked bore creates more friction, accelerating erosion both due to melt-wipe and pieces of cracked steel breaking off. Mechanical erosion, however, is definitely in third place as a cause of barrel burnout, far behind thermal and chemical erosion in all but very low-pressure cartridges. Obviously, the easiest way to avoid burning out a barrel is not to shoot quickly and repeatedly, thus avoiding heat build-up. This is exactly why many prairie dog shooters take more than one rifle afield. When the barrel on one rifle starts to heat up, they put it in the shade to cool off, with the barrel pointing up and the action open. On a really hot day some shooters drape a damp towel around the barrel, allowing the rifle to get back into action sooner. It also helps to shoot smaller cartridges for a given bore-size, one reason larger .22 centerfires like the .22250 and .220 Swift have become less popular for prairie dog shooting. On a big prairie dog town the barrel of a .22250 can be totally fried in one day of consistent shooting. On a hot day in western Kansas one of my companions shot his walnut 52 W W W. G U N S M A G A Z I N E . C O M • D E C E M B E R 2 0 1 2